M. A. Fahnestock
Our knowledge of the large ice sheets has improved dramatically over the last 15 years, in large part due to satellite observations. Sensors ranging from low and high resolution visible and near-IR to active and passive microwave have allowed studies in areas that are essentially unmapped, having been traversed only sporadically during scientists' limited 100 year history in the polar regions. While travel in the polar regions has improved, and could even be called comfortable, surface measurements are still restricted in scope, and can only be extrapolated or interpolated effectively with the use of satellite data.
Many recent studies have focused on the roles of rapid ice discharge and melt in determining the fate of the ice sheets. Current measurements place the balance of mass in different areas both in the positive and the negative, but the errors and the signal are of the same magnitude. The question of ice sheet mass balance has global consequences, as sea level will be directly effected by any change in the volume of the ice sheets.
The longest historical record of ice sheet and glacier conditions has been the observation of the position of the terminus or edge of the ice sheet. Such records can reach back several hundred years in populated areas, but in many places such records exist only from what can be seen with Landsat or SPOT. The use of change in margin position as an indication of changing climate is complicated by the different response timescales of the atmosphere and ice flow, which is regulated both by the amount of ice and by the thermal and flow history of that ice.
Rapidly flowing outlet glaciers and ice streams provide a mechanism by which ice sheets can change configuration rapidly, and may even provide a route for "collapse." Studies of these features has accelerated with the use of satellites for measuring ice motion. Ice motion can be measured in several ways. Techniques which use visible imagery (typically TM and SPOT) track crevasses and other ice surface features from one image in a second, later image. The images need to be co-registered using features that do not move - this can be difficult, but not impossible, in a fully ice-covered scene. The digital version of feature tracking uses simple cross-correlation, and has an overall accuracy of about half of the resolution.
A second, more recently developed motion measurement technique, uses interferometry from synthetic aperture radar (SAR) imagery to measure motion and detailed ice surface topography. This technique has broad application where adequate data is available, and may be used to estimate the full vector motion field. It has been used to measure ice discharge in featureless areas, determine grounding line positions, and to detect variable flow behavior in an outlet glacier. Interferometry suffers from atmospheric artifacts and long-wavelength errors, but offers results that cannot be obtained in any other way.
Direct measurement of ice sheet volume has been a goal of radar altimetry since the launch of SEASAT in 1978. The radar altimeters that have been flown have all been primarily designed for oceanographic measurements, and have difficulty tracking the rapidly changing slopes that are found around the periphery of ice sheets. The radar altimeters on ERS-1 and ERS-2 had a second tracking mode that improved the situation in these areas, but the greatest advance in the field came from the increased coverage provided by their high inclination orbits. Radar altimeters have another limitation, which is the size of their footprint and the low curvature of the wavefront at the ice surface. These instruments are limited in their ability to return the detailed local topography that is present on ice sheets. This detailed topography is visible in AVHRR and Landsat images, and can be measured with photoclinometry and interferometry.
The problems with radar altimetry will in large part be solved by laser altimetry from space. The GLAS instrument, which is scheduled to fly early in the next century, will provided an improved record of ice surface elevation. It is complemented by aircraft based laser altimetry in Greenland, which will provide a 5 year baseline change measurement of that ice sheet in the next two field seasons.
Passive microwave sensors are used to monitor the spatial extent and duration of melt events in the snowpack. This data, with complete coverage on a daily basis, can also be used to provide information about the timing of events that are recorded in the snowpack, and can allow extrapolation of surface temperature measurements away from automatic weather stations located on the ice. In addition, visible and IR imagery can be used to estimate albedo, which changes dramatically with the onset of melt.
Contact Information:
Mark A. Fahnestock
Earth System Science Interdisciplinary Center
Dept. of Meteorology
Computer and Space Sci. Bldg.
University of Maryland
College Park MD 20742
Telephone: (301) 405-5384
email: mark@atmos.umd.edu